Background: Traditional agriculture is on the front line of climate change, being most impacted by the increase in the intensity and frequency of extreme events, such as floods, drought and rising temperatures. Local ecological knowledge is a recognized keystone of successfully managed socioecological systems, but loss of soil fertility, water scarcity, incidence of diseases and decreased production due to climate change are linked to the greater vulnerability experienced by traditional farmers. Plant biostimulants are natural products used to stimulate nutrient uptake and efficiency by crops, increase tolerance to abiotic/biotic stress and improve quality without negative impacts on the environment if obtained from renewed sources. Humic substances are some of the most used plant biostimulants in agriculture and play a central role in plant adaptation. Materials and methods: We reviewed and discussed a sample set of papers (n = 52) about humic substances to mitigate abiotic stress in crops using data basis from Web of Science (Clarivate Analytics), Scopus—IBM (International Business Machines Corporation), and Scielo (Scientific Electronic Library Online). Results: The predominance of authors in the global south is notable, but it is not a coincidence, since this is where the effects of climate change will have the greatest impact. The mechanisms involved in the stress mitigation involve the activation of signaling factors, gene response induction, the accumulation of osmoprotective and anti-oxidant compounds, the induction of antioxidative metabolism, ion homeostasis, membrane transport and adjustment of hormonal balance. The intriguing question is: how can a complex mixture of molecules affect so many distinct effects on plants responsible for plant adaptation? Conclusions: The complexity of humic substances challenges our knowledge method, but supramolecular chemistry may provide answers that enable us to broaden our understanding of the plant defense mechanisms modulated by these substances. Graphical

Intensive agriculture maintains high crop yields through chemical inputs, which are well known for their adverse effects on environmental quality and human health. Innovative technologies are required to reduce the risk generated by the extensive and harmful use of pesticides. The plant biostimulants made from humic substances isolated from recyclable biomass offer an alternative approach to address the need for replacing conventional agrochemicals without compromising the crop yield. The stimulatory effects of humic substances are commonly associated with plant hormones, particularly auxins. However, jasmonic acid (JA) is crucial metabolite in mediating the defence responses and governing plant growth and development. This work aimed to evaluate the changes in the biosynthesis and signalling pathway of JA in tomato seedlings treated with humic acids (HA) isolated from vermicompost. We use the tomato model system cultivar Micro-Tom (MT) harbouring a reporter gene fused to a synthetic promoter that responds to jasmonic acid (JERE::GUS). The transcript levels of genes involved in JA generation and activity were also determined using qRT-PCR. The application of HA promoted plant growth and altered the JA status, as revealed by both GUS and qRT-PCR assays. Both JA enzymatic synthesis (LOX, OPR3) and JA signalling genes (JAZ and JAR) were found in higher transcription levels in plants treated with HA. In addition, ethylene (ETR4) and auxin (ARF6) signalling components were positively modulated by HA, revealing a hormonal cross-talk. Our results prove that the plant defence system linked to JA can be emulated by HA application without growth inhibition.

The objective of this study was to evaluate soil macrofauna as a bioindicator of soil quality in successional agroforestry systems and secondary forests. The study was conducted in the southern lower region of Bahia in Brazil, in two areas: a successional agroforestry system (AFS18) and native forest (NF). AFS18 consists of two species: mahogany (Khaya ivorensis and Khaya grandifoliola), açaí (Euterpe oleracea), cacao (Theobroma cacau) and banana (Musa spp.). Sampling was carried out in the dry (June) and rainy (October) seasons of 2019, and eight soil monoliths were collected in both areas. A total of 889 individuals from the soil macrofauna were sampled. The highest frequency (RF) of taxons occurred in NF in the rainy season, and the groups that stood out were: Oligochaeta with 42% FR in ASF18, Formicide with 33.9% in NF and Isoptera with 58% in AFS18. The macrofauna structure of the soil varied according to the time of collection. The density of macrofauna individuals differed between areas only in the dry season. The highest number of ind.m² was observed in the area NF (378) when compared to ASF18 (196). TOC, Mg2+, Al3+ and CTC were related to AF on both occasions of collection and AFS18 in the rainy season, K+, P and pH were associated with AFS18 in the dry season. The diversity, equitability and richness of the soil macrofauna was greater in AF area. HFA18 in the rainy season was similar to NF, favoring colonization of the area by soil macrofauna organisms.

The effect of humic substances (HSs) in combination with or without plant-growth-promoting bacteria on soybean vegetative growth and root nodulation was examined in this study. Seeds were inoculated with Bradyrhizobium japonicum SEMIA 5079 in the presence of HSs from leonardite and Herbaspirillum seropedicae HRC54. Additional HSs and H. seropedicae application at the substrate surface was conducted at the V3 stage. The experiment was carried out in a greenhouse using pots filled with a top layer of an Oxisol soil, and plants were harvested at the R1 stage. The HS and H. seropedicae treatments significantly promoted plant shoot and root growth. The number and weight of soybean nodules were higher in the treated plants when compared to a control. The plant nodulation process was affected by the treatments that included activities of malate dehydrogenase (MDH), nitrate reductase (NR) and plasma membrane H⁺-ATPase (MHA). At low concentrations, the HSs and H. seropedicae increased the nodule quantity, size and weight, favoring plant growth. Combining humic substances and plant-growth-promoting bacteria (PGPB) could be a promising approach to promoting soybean nodulation and increasing crop production.

Plant biostimulants (PBs) have been considered the new wave for ecological intensification and sustainability, but are they sustainable? They increase nutrient use efficiency and reduce the impact of abiotic stress in plants. However, commercially available PBs based on humic substances are obtained using non-renewable sources of organic matter. At the same time, the microbial inoculants include a discussion of the properties of microorganisms and formulation design, as well as standards of purity and process control. Farmers depend on biological inputs like others to generate additional income for agribusiness. We produced a composite PB using humic substances isolated from vermicompost with KOH 5% and microbial consortia of plant growth-promoting bacteria (H. seropedicae, G. diazotrophicus, Bacillus spp.) grown in a simple medium with molasses and fishmeal as sources of C and N, respectively, in a homemade reactor at 37 °C for 36 h. The on-farm PB was applied directly in a passion fruit trial, and plant health and yield were monitored. The plants treated with the PB showed decreased visual symptoms of pests and diseases concurrent to higher activities of the enzymes used to monitor the induction of the plant resistance system (1,3-β glucanase, peroxidase, and phenylalanine ammonia-lyase). Plants treated with the PB yielded more than 50% more passion fruit than the control in soil with natural low fertility, fertilized with vermicompost. It is possible to produce PBs directly on the farm, leveraging locally available resources and simple technologies to sustainably enhance plant health and productivity.

Modern agriculture relies heavily on chemical inputs to sustain productivity, yet their intensive use poses environmental and health risks. Sustainable strategies based on biostimulants have emerged as promising alternatives to reduce agrochemical dependence. Among these compounds, humic substances (HS) stand out for their ability to modulate plant growth and activate defense responses. This study aimed to evaluate the effects of HS from different sources—vermicompost (Vc) and peat (Pt)—on the salicylic acid (SA)-mediated defense pathway in tomato plants (Solanum lycopersicum cv. Micro-Tom) infected with Oidium sp. The HS were characterized by solid-state ¹³C CPMAS NMR to determine the relative distribution of carbon functional groups and structural domains, including alkyl, O-alkyl, aromatic, and carbonyl carbon fractions, as well as hydrophobicity-related indices. Enzymatic activities of lipoxygenase, peroxidase, phenylalanine ammonia lyase, and beta 1,3-glucanase were determined spectrophotometrically, and RT-qPCR quantified gene transcription levels involved in SA signaling and defense (MED25, MED16, MED14, NPR1, ICS, PAL, LOX1.1, MYC2, JAZ, jar1, CAT, POX, SOD, APX, ERF, PR-1, PR-2, PR-4 e PR-5). Both HS significantly reduced disease severity and activated key SA-related defense genes, including the regulatory gene NPR1 and the effector genes PR1, PR2 and PR5, with Pt providing greater protection. Notably, HS amplified defense-related gene expression and enzymatic activities specifically under infection, showing a stronger induction than in non-infected plants. These results demonstrate that structural differences among HS drive distinct and enhanced defense responses under pathogen challenge, highlighting their potential as sustainable tools for improving plant immunity in agricultural systems.